A conventional image-forming apparatus, such as a laser printer and an electro copying machine, has a construction, for example, as shown in FIG. 29. In this image-forming apparatus, the formation of an electrostatic latent image is carried out on a photoreceptor drum 101 by uniformly charging the surface of a photoreceptor drum 101 by a charger 102 and then partially reducing the electric potential of the surface by exposure carried out by an exposure device 103. In this case, an aluminum element cylinder (not shown in the Figure) forming a base body of the photoreceptor drum 101 is connected to ground through the drum flange and the drum shaft; thus, it is possible to partially reduce the electric potential at the exposed portions on the photoreceptor drum 101.
Moreover, the above-mentioned electrostatic latent image is developed by applying a developing bias to a developing device 104 so as to allow developer to adhere to the electrostatic latent image in the developing device 104.
Normally, a scorotron charger is used as the above-mentioned charger 102. As illustrated in the same Figure, this charger is provided with a control grid 102b which can control the quantity of corona ions that reaches the surface of the photo receptor drum 101 from the discharge wire 102a.
The image-forming apparatus is further provided with a transfer device 105, a separation device 106, cleaning device 107 and a static eliminating lamp 108.
In the conventional image-forming apparatus, when no electric potential is applied thereto by the charger 102, the surface electric potential of the photoreceptor drum 101 is maintained at 0 V. For this reason, in an image-forming apparatus using a reversal developing system, during non-image-forming periods such as the initial setting time, a warming up period, a printing start time or a printing end time, if the developing device 104 or the photoreceptor drum 101 is rotated, developer tends to adhere to the photoreceptor drum 101, thereby causing the developer to be unnecessarily consumed. In order to prevent such a problem, electric potentials are applied to both of the photoreceptor drum 101 and the developing device 104 even during the above-mentioned non-image-forming periods.
For example, Japanese Laid-Open Patent Application No. 282126/1994 (Tokukaihei 6-282126), published on Oct. 7, 1994) has proposed a construction in which, during an unstable rotation period at the time of starting the main motor, an electric potential having the same polarity as the developer is applied to an area of the photoreceptor on which the developing device acts. For example, in the apparatus shown in FIG. 29, a high-voltage transformer power supply 109 is connected as the power source for the control grid 102b. In this construction, during the non-image-forming period, the respective sections are operated in synchronized timing as shown in FIG. 30. Here, the drum motor is a driving motor for the photoreceptor drum 101, and the laser light emission is a releasing operation of a laser beam from the laser device installed in the exposure device 103.
Here, in the high-voltage transformer power source 109, at the time of a rise in voltage, an unstable area appears, which is an area in which the voltage becomes uncontrollable at the leading-edge on the low voltage side. As shown in FIG. 31, because of this unstable area, the surface electric potential of the photoreceptor is not allowed to rise linearly, and has a distortion including so-called rounding 111. For this reason, during the rising process of the surface electric potential of the photoreceptor to a predetermined electric potential, the electric potential difference (fog electric potential) between the surface electric potential of the photoreceptor and the developing bias becomes excessively greater. As a result, scattering of the carrier (carrier rise) from the developing device 104 occurs, resulting in problems such as short service life of the photoreceptor in the photoreceptor drum 101, formation of toner aggregation with scattered carrier serving as cores at the time of recycling the toner, and the subsequent degradation in copied images.